Integrated time dependent dielectric breakdown reliability testing

ABSTRACT

Systems for reliability testing include a picometer configured to measure a leakage current across a device under test (DUT); a camera configured to measure optical emissions from the DUT based on a timing of the measurement of the leakage current; and a test system configured to apply a stress voltage to the DUT and to correlate the leakage current with the optical emissions using a processor to determine a time and location of a defect occurrence within the DUT by locating instances of increased noise in the leakage current that correspond in time with instances of increased optical emissions.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.:N66001-11-C-4104 (awarded by Defense Advanced Research Projects Agency(DARPA)). The government has certain rights in this invention.

BACKGROUND

Technical Field

The present invention relates to reliability testing and, moreparticularly, to isolating and localizing time dependent dielectricbreakdown defects.

Description of the Related Art

Time dependent dielectric breakdown (TDDB) in the back-end-of-line(BEOL) in integrated circuits is a significant source of reliabilityproblems as circuit formation technology reaches 22 nm and beyond. As aresult, the performance of interconnection is susceptible to technologyshrinking, new material (low-k value) features, and process improvementand development. To better understand the effect, leakage currentmeasurement and emission microscopy tests have been conductedseparately. Leakage current measurement demonstrates the evolution ofdielectric breakdown times. Extrapolation and interpolation on themeasurement results then enable the lifetime analysis of the dielectricand the TDDB effect. Light emission tests have also been used, sincephoton emission is recognized as occurring via energy states generatedby dangling bonds and/or impurities at the material interface, which istightly related to the TDDB progressive development. However, thedetailed mechanism of the TDDB effect is still not clear, for that: (1)the progressive development of TDDB effect is not carefully caughton-site; and (2) all prior analysis was conducted off-line and after theexperiments, when the TDDB sites on the device under test (DUT) aretotally destroyed. This leads to inaccuracy and insufficient for thefurther failure analysis, including physical failure analysis andscanning electron microscope; and (3) the correlation between electricalleakage current and photon emission is not studied.

SUMMARY

A system for reliability testing includes an electrical measurementdevice configured to measure an electrical characteristic of a DUT; acamera configured to measure an optical characteristic of the DUT basedon the timing of the measurement of the electrical characteristic; and atest system configured to apply a stress to the DUT and to correlatemeasurements of the electrical characteristic with measurements of theoptical characteristic using a processor to determine a time andlocation of a defect occurrence within the DUT.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a pair of graphs that show electrical measurements of acircuit undergoing a time-dependent dielectric breakdown (TDDB);

FIG. 2 is a diagram of an exemplary device under test comparing an imagebefore a TDDB event and an image during the TDDB event;

FIG. 3 is a diagram of a system for performing TDDB reliability testsaccording to the present principles;

FIG. 4 is a block/flow diagram of a method for integrated electrical andoptical testing according to the present principles;

FIG. 5 is a set of graphs showing a correlation between electrical andoptical measurements according to the present principles; and

FIG. 6 is a test system to control and analyze TDDB testing according tothe present principles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present principles provide reliability testing that combines boththe electrical and emission characteristics of a device under test.Electrical monitoring of the DUT is performed during various types ofstress in order to detect TDDB. The present principles provideintegration and synchronization of electrical measurements and emissionmicroscopy with an online instantaneous data analysis and furtheroffline data analysis and processing. This provides early detection ofTDDB events and also allows precise spatial localization before any realdestructive event takes place, so that physical failure analysis can beperformed to investigate precursors and root causes that lead to theTDDB event. This is contrasted to previous techniques, whereby emissiontesting was only performed post-measurement in a failure analysis modeto aid in the localization of the destroyed region of the DUT.

Referring now to FIG. 1, two graphs are shown that show electricalmeasurements of a circuit undergoing a TDDB event. Graph 102 has avertical axis of voltage, measured in volts, and a horizontal time axismeasured in hours. The graph 102 shows a linear voltage increase withrespect to time. Graph 104 has a similar horizontal time axis, butmeasures leakage current on its vertical axis in amperes. At time 106,abnormal leakage current fluctuations begin in the DUT. Thesefluctuations represent an in-progress TDDB event. Eventually, at time108, the DUT has been destroyed by TDDB effects, producing a stable, butmuch higher, leakage current. The characteristic breakdown pattern shownin graph 104 will not precisely reflect every type of technology, but asimilar pattern may be generated for any type of TDDB breakdown event.Noisy behavior, such as that shown in graph 104 after point 106, isfrequently a signature of TDDB events.

It should be noted that the TDDB event may occur at any point on theDUT, such that it is impossible to know from the electrical measurementsalone precisely where the breakdown happened. There is no externalvisual indication to show the breakdown after it has occurred.

Referring now to FIG. 2, an exemplary DUT 202 is shown at two differentpoints in time. The representation of the DUT 202 is a top-down picture,taken over a period of time to allow accumulation of emission light. Att=5 hours, no emissions are visible at the DUT 202. However, during theTDDB effect, at t=5.7 hours, a point of light 204 is recorded by thecamera indicating the physical location of the breakdown in the DUT 202.

There are many effects which can cause point emissions such as thatshown as 204. As such, merely providing visual measurements of a DUT 202does not suffice to determine which points represent TDDB events.However, by integrating electrical measurements and opticalmeasurements, the characteristic breakdown pattern shown in graph 104can be used to provide timing information for a camera, such thatoptical emissions 204 can be correlated with known TDDB events.

Referring now to FIG. 3, a system 300 for performing TDDB reliabilitytests is shown. A DUT 302 is optically monitored using an appropriateimaging camera 304. This camera may be any suitable emission microscopysystem with a sufficiently high accuracy and resolution to detect theemissions produced by a TDDB event. Any camera 304 will have adetermined exposure time needed to detect an emission event. In oneembodiment, the camera 304 may detect emissions in near-infra redwavelengths, but it is also contemplated that other tools such as asuperconducting quantum interference device (SQUID), thermal cameras,and laser stimulation tools may be employed to localize TDDB sites.

Exposure time for the camera 304 represents a period of integration thatdetermines the time resolution of optical imaging. A camera 304 whichintegrates over the entire duration of a test will record every emissionevent, but will not be able to distinguish said events in time. As such,it is advantageous to limit the integration period to a minimal lengthof time that enables detection. Alternatively, the camera 304 may beused in a “movie” mode, which may capture faster changing events if saidevents are relatively bright. In each case, a high spatial resolution ofimages permits a concurrent precise spatial localization of any detectedemission to guide subsequent physical failure analyses.

An electrical measurement device 306 monitors electrical properties ofthe DUT 302 at a known frequency. It is specifically contemplated thatthe electrical measurement device 306 may be a picometer measuringleakage currents, but this is not intended to be limiting. Anyappropriate measurement device may be used to detect characteristic TDDBpatterns. The camera 304 and the measurement device 306 providemeasurement information to the test system 308. The test system 308coordinates measurements from the measurement device 306 with imagingperiods in the camera 304. After every electrical measurement, the testsystem 308 initiates a new emission test if the camera is idle. Thecorrelation between specific measurements and particular emission testsis stored in a memory in the test system 308 for analysis.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing. Computer program code for carrying out operations foraspects of the present invention may be written in any combination ofone or more programming languages, including an object orientedprogramming language such as Java, Smalltalk, C++ or the like andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The program codemay execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, includinga local area network (LAN) or a wide area network (WAN), or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks. The computer program instructions may also beloaded onto a computer, other programmable data processing apparatus, orother devices to cause a series of operational steps to be performed onthe computer, other programmable apparatus or other devices to produce acomputer implemented process such that the instructions which execute onthe computer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. The flowchart and block diagrams in the Figuresillustrate the architecture, functionality, and operation of possibleimplementations of systems, methods and computer program productsaccording to various embodiments of the present invention. In thisregard, each block in the flowchart or block diagrams may represent amodule, segment, or portion of code, which comprises one or moreexecutable instructions for implementing the specified logicalfunction(s). It should also be noted that, in some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

Referring now to FIG. 4, a block/flow diagram of a method for integratedelectrical and emission testing is shown. Block 402 begins byinitializing test parameters. These parameters may include, for example,stress voltage, stress duration, an overall testing time, a camera mode,a camera integration period, an electrical measurement frequency, andearly-termination criteria. These parameters may be entered manually attest system 308 or loaded from predetermined configuration files.Reading in a configuration file offers greater flexibility incontrolling measurements, allowing one to create highly robustmeasurement schemes. For example, stress voltage may be specifiedaccording to a designated waveform.

Block 403 begins stressing the DUT 302 by applying, e.g., a stressvoltage. As noted above, the particular voltage pattern and duration maybe set as user parameters, but it is particularly contemplated that thestress voltage may be a linearly increasing function of time.Optionally, the stress test may run for a predetermined period of time,because TDDB events frequently take time before a breakdown occurs.Thus, to save on storage space, it may be advantageous to delaycollection of emission data until TDDB events can be more reasonablyexpected.

Block 404 then begins electrical measurement and optical emissiontesting. As noted above, electrical testing may include periodicallymaking electrical measurements of the leakage current across the DUT 302using, e.g., a picometer 306. After each electrical measurement, block404 checks the status of camera 304: if the camera is idle, a newemission test is invoked, but if the camera is busy, block 406 idlesuntil the emission test is finished.

After the emission test completes, block 408 performs analysis on thecollected data. In the case of a camera 304 in single image mode,emission information is integrated over a specified duration of time toproduce a final image. Data analysis may include, e.g., digital1-dimensional and 2-dimensional filters, derivatives, 2-dimensionalgradients, and correlation functions to detect early signs of a TDDBevent. These early signs may include increases in leakage current andthe appearance of emission spots. A time stamp is associated with eachelectrical measurement, and each emission measurement may include astart/stop time. Additional associated information that may be storedwith a given measurement includes a time difference from a firstmeasurement and a present stress voltage value.

These analytical operations accomplish several goals. Signatures ofearly TDDB initiation may be used to adaptively change the emissioncomponent of the experiment, such as changing parameters that mayinclude acquisition time, acquisition rate, and single-image vs. moviemode. They may also be used to start or stop acquisition. Signatures inboth the optical and electrical realms (or a combination of the two)could be used for early detection of the formation of a TDDB event tomodify the stress experiment parameters, for example slowing down theprocess to highlight particular physical phenomena, or even to stop thestress process before the DUT is destroyed, so that physical failureanalysis can identify the precursors of TDDB. Analysis can be used tospatially localize the position of the TDDB defect, before or after adestructive event takes place, for later physical failure analysis.Furthermore, by studying the progression of the TDDB effect from itsearly formation through the destruction of the structure and beyond, aTDDB event in one spatial location may be observed as being followed byother TDDB events at different locations. The region close to apreviously damaged location may be susceptible to additional TDDBevents.

If data analysis concludes that some specified termination criterion hasbeen met at block 410, e.g., if a TDDB event has been detected,processing halts. If not, and if an overall testing time has not yetelapsed at block 412, processing returns to block 404 and a new set ofelectrical and emission tests begins. If the overall testing time haselapsed, processing ends.

The data analysis of block 408 includes establishing correlationsbetween electrical measurements and optical emission data. Historicalvalues of electrical measurements may be formatted into an electricalmeasurement vector. When measuring, for example, leakage current, aleakage current vector is formed at each measurement time instant m thatincludes all past measurements: I=[I₁, I₂, I₃, . . . , I_(m)].Meanwhile, historical values of optical measurements are also formattedinto an optical measurement vector: E=[E₁, E₂, E₃, . . . , E_(n)]. Thelength of the I and E vectors are usually different, withlength(I)≠length(E), so a convolution is used to determine thecorrelation between them. The correlation between the two vectors iscalculated as

${{Con}_{k} = {{{conv}\left( {I,E} \right)} = {\sum\limits_{j}\; {{E(j)}{I\left( {k + 1 - j} \right)}}}}},$

where k=1, . . . , m+n−1 and where the maximum value is regarded as themaximum correlation between the electrical and optical measurements:Corr=max (Con_(k)). This is only one possible way to correlate the twovectors. Any appropriate correlation may be used to, e.g., find the besttime to stop testing to capture the TDDB event.

In the above evaluation of the correlation between electrical andoptical measurements, the maximum emission intensity in the chip area isused to compose the emission vector. While this value may be quicklycalculated, it is not able to differentiate between locations. For moreaccuracy, the chip area can be divided into sub-areas or divided intopixels. Then, multiple emission vectors, each representing a differentarea in the chip image, may be used to correlate with the singleelectrical measurement vector. The sub-area having the highest emissionvalue is used as the maximum emission intensity.

Referring now to FIG. 5, graphs showing measurements of leakage current502, emission intensity 504, and the calculated correlation 502 betweenthe two is shown. As can be seen from graph 504, emission events aredetected which are clearly unrelated to the TDDB event shown in graph502. The increase in leakage current shown in graph 502 corresponds witha specific emission event in graph 504, producing a corresponding jumpin the calculated correlation shown in graph 506.

Referring now to FIG. 6, a more detailed view of testing system 308 isshown. It should first be noted that, although testing system 308 isshown as one unit, its functions may be divided into multiple differentdevices. This may be advantageous in circumstances where, for example,data analysis might slow the acquisition of data from tests. Byimplementing such functions on separate hardware, testing efficiency canbe increased.

Testing system 308 includes a processor 602 to perform processingrelated to data analysis and test control, as well as a memory 604 tostore measurement and configuration information. The processor 602 andmemory 604 are controlled by functional modules which, as describedabove, may be implemented as hardware, software, or a combination of thetwo. A configuration module 606 loads in testing parameters, whetherinputted manually or by reading a configuration file from memory 604.Said parameters may include, e.g., stress voltage, stress duration, anoverall testing time, a camera mode, a camera integration period, anelectrical measurement frequency, and early-termination criteria.

A testing module 608 initiates and controls electrical and emissiontesting by controlling, e.g., electrical measuring device 306 and camera304. The testing module 608 monitors the status of the measuring device306 and the camera 304 to determine when said devices are active or idleand coordinates the activation of the respective testing cycles. Thetesting module 608 further collects data from each respective device andstores said data in memory 604.

A data analysis module 610 accesses data stored in memory 604 and usesprocessor 602 to analyze the measurements as set forth above. Inparticular, the data analysis module 610 attempts to correlateelectrical and optical measurements to predict and localize TDDB events.The data analysis module 610 may furthermore communicate withconfiguration module 606 and testing module 608 to provide realtimechanges to parameters and testing procedure in response to detectedconditions. For example, in one case the data analysis module 610 mayhalt testing upon detection of the early stages of a TDDB event, suchthat later physical failure analysis may locate precursors to TDDBfailures. A display module 612 displays real-time or offline results andanalysis, allowing human operator to further control the testingprocedure.

The testing system 308 furthermore has a DUT interface 614, which allowstesting module 608 to provide, e.g., stress voltage to the DUT, and ameasuring interface 616 that allows the testing module 608 tocommunicate with the electrical measurement device 306 and the camera304. It should further be recognized that the testing system 308 mayinclude additional interfaces and modules to enable subdivision of thefunctions of the system, for example by splitting testing and analysisfunctions into separate devices. In such a case, additional processors602 and memory 604 may be employed to prevent, e.g., analysis frominterfering with measurements.

Having described preferred embodiments of a system and method forintegrated time-dependent dielectric breakdown reliability testing(which are intended to be illustrative and not limiting), it is notedthat modifications and variations can be made by persons skilled in theart in light of the above teachings. It is therefore to be understoodthat changes may be made in the particular embodiments disclosed whichare within the scope of the invention as outlined by the appendedclaims. Having thus described aspects of the invention, with the detailsand particularity required by the patent laws, what is claimed anddesired protected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A system for reliability testing, comprising: anelectrical measurement device configured to measure an electricalcharacteristic of a device under test (DUT); a camera configured tomeasure an optical characteristic of the DUT based on the timing of themeasurement of the electrical characteristic; and a test systemconfigured to apply a stress to the DUT and to correlate the electricalcharacteristic with the optical characteristic using a processor todetermine a time and location of a defect occurrence within the DUT. 2.The system of claim 1, wherein the test system is configured to convolvea vector formed from electrical measurements with a vector formed fromoptical measurements.
 3. The system of claim 2, the test system isconfigured to calculate a maximum value of the convolution as${{Corr} = {\max\left( {\sum\limits_{j}\; {{E(j)}{I\left( {k + 1 - j} \right)}}} \right)}},$where k=1, . . . , m+n−1, I is the optical measurement vector, and E isthe electrical measurement vector.
 4. The system of claim 1, wherein thedefect is a time dependent dielectric breakdown.
 5. The system of claim1, wherein the stress is a stress voltage applied across the DUT.
 6. Thesystem of claim 5, wherein the applied stress voltage increases linearlywith time.
 7. The system of claim 1, wherein the electricalcharacteristic is a leakage current.
 8. The system of claim 1, whereinthe optical characteristic is an optical measurement of emissions fromthe DUT over a specified period of time.
 9. The system of claim 1,wherein the test system is further configured to adjust parameters ofsaid stress, said electrical measurement, or said optical measurement,based on the correlation.
 10. The system of claim 1, wherein the testsystem is further configured to halt said stress, electricalmeasurement, and optical measurement, based on the correlation.
 11. Thesystem of claim 1, wherein electrical measurement device is furtherconfigured to periodically repeat the electrical measurement.
 12. Thesystem of claim 1, wherein the test system is further configured tolocate instances of increased noise in the electrical measurement thatcorrespond in time with instances of increased emissions in the opticalmeasurement.
 13. The system of claim 1, wherein the test system isfurther configured to segment an image of the DUT and determine amaximum emission intensity for each segment to localize emissions.